If you look in the plug hole and things are not obviously destroyed, the next step is to take the valve cover off and see if you can find all the missing pieces. The top washer should be in there somewhere, but the other collet is small enough to have gone down one of the oil drain holes or perhaps by one of the pushrods. If you sre lucky it will have made it all the way down to the pan and come out when you drain the oil.

Then examine the top end of the valve stem very closely for any imperfections in the collet grooves. Also make sure the valve moves up and down freely and doesn't wobble from side to side. If it looks good, put everything back together with new collets. If the valve spring is bent or otherwise damaged from rattling around among the moving parts, replace it too. Stuff rope in the cylinder through the plug hole and turn the crankshaft to hold the valve closed. Finish putting it back together and check the compression in that cylinder. If it's OK, then see how it runs.

If there is obvious damage when you look in the plug hole, or any damage at all to the top end of the valve stem, or little or no compression with it back together (suggesting the valve is bent) the head will have to come off.